The present invention relates generally to applying solder to a substrate and, more particularly, to the selected placement of solder using a solder jet.
Depositing solder selectively on a substrate is well known. Different techniques include stenciling or screening a solder paste onto the substrate, using solder balls selectively placed where metal contact is desired, and chemically vapor depositing the metal onto the surface of the substrate. Each one of these methods has advantages and disadvantages.
The use of a stencil to fabricate a conductive trace pattern on the surface allows for precise alignment and placement of the solder. Unfortunately, the stencils are expensive to design and produce and they wear out after repeated use. When they wear out, solder seeps through the worn stencil areas across those areas where no solder is desired, causing shorts, or no solder is being placed where it is needed, causing a breach or open connection. These areas have to be repaired and if these types of conditions are repeated with any type of frequency, the stencil must be replaced with a new stencil. Additionally, stencils require periodic cleaning, which adds another processing step to clean the stencil as well as lessens the useful life of the stencil.
The use of solder balls has been a tremendous advance in the art of electrically connecting a device to the surface of a printed circuit board. Solder balls, however, have quality control problems as their critical dimensions continue to decrease. The ability to produce balls of the same diameter consistently decreases as the diameter decreases. Thus, for some diameters of solder balls, the range of acceptable product can be solder balls having diameters more than twice the desired diameter. Or, they can have diameters half the size of the desired diameter. This requires that the tolerances at the surface contact level of a substrate, such as a semiconductor device, must allow for a solder ball having a diameter that is from 50% smaller to 100% larger than the specified size. Further, working with solder balls is difficult because of their size and the methods needed to place them accurately. When they fail to be placed accurately, or are missing entirely, problems occur in the resulting assembly of a semiconductor device attached to a substrate that must be corrected. These problems include shorts or opens that must be fixed. No easy solution yet exists for repairing missing or improperly sized solder balls after a semiconductor device has been mechanically attached in place on a substrate.
Chemical vapor deposition (CVD) allows for precise alignment of conductive traces and for batch processing. CVD does have limitations however. These limitations include being unable to place the package directly on the surface of the printed circuit board (PCB) immediately after depositing the metal on the surface since a cooling step is typically needed. Further, clean conditions are always necessary when using CVD, which requires expensive equipment and control. Additionally, when clean conditions do not exist, shorts or opens in assemblies can occur that need to be repaired once they are discovered.
A new approach to deposit solder on a surface, such as a printed circuit board (PCB), is to deposit the solder using a solder jet, similar to the manner in which ink jets deposit ink onto paper for printing. The ink jet technology is well established, but due to different problems associated with solder, ink jet technology is not directly applicable to solder jet technology. For example, solder jets use molten melt as a print agent, whereas ink jets use heated water-based ink. Since the print agent is metal in solder jets, the viscosities and densities are much different as are the operating temperatures. Thus, applying ink jet solutions to solder jet problems is impractical.
One typical solder jet apparatus has recently been developed by MPM Corporation. The solder jet apparatus takes liquid solder and forms it into a stream of droplets that have a uniform size and composition. The formation of the droplets involves generating a consistent pressure coupled with a vibration force sufficient enough to dislodge the drops from the jet nozzle in a steady state with a uniform size and consistency. Once the solder droplets are formed, gravity forces them downward where they impact on the surface of the substrate. The solder droplets pass through a charging electrode to impart a charge on the metal droplets.
The system operates using a binary control that either allows the droplets to impact on the surface or to be removed into a droplet catcher for recycling when no droplets are desired. Since the droplets were charged at one point, an electric field or pulse can be asserted, causing the droplets to either continue to the surface or to fall into the catcher. With this system, the exact position of the droplets is known and never varies. Thus, the substrate must be moved to the desired grid for the droplets to impact the area desired to be soldered. This results in a highly inefficient system since the substrate must be stopped for each application of solder to a new location. This also involves greater mechanical complexity since the table holding the substrate, or the solder jet apparatus itself, must be moved and aligned properly before solder can be deposited.
Accordingly, what is needed is a solder applicator that allows for greater precision in placing the droplets along with increased efficiency in product throughput.